X-ray generating tube, x-ray generating apparatus and x-ray imaging  system using the same

ABSTRACT

Provided is an X-ray generating tube with improved withstand voltage property by a simple structure, the X-ray generating tube including a cathode connected to one opening of an insulating tube and an anode connected to the other opening, in which a resistive film having a lower sheet resistance value than that of the insulating tube is disposed on an outer periphery of the insulating tube, and the cathode and the anode are electrically connected to each other via the resistive film.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an X-ray generating tube, and an X-raygenerating apparatus and an X-ray imaging system including the X-raygenerating tube, which can be used for a medical equipment, anondestructive inspection apparatus, and the like, for example.

2. Description of the Related Art

In general, an X-ray generating tube accelerates electrons emitted froman electron emitting source with use of a tube voltage in a vacuum andirradiates a target made of a metal such as tungsten with the electronso as to generate X-ray. The tube voltage set in the X-ray generatingtube is approximately 100 kV, for example, and the X-ray generating tubeis required to have a structure that can withstand the high voltage.

In Japanese Patent Application Laid-Open No. 2009-245806, there isdisclosed a technology for stably suppressing breakdown of a valve dueto a discharge by applying an electrical resistive film onto an outersurface of the valve as an electrical insulation portion, in which theelectrical resistive film is electrically connected to a case main bodyas an electrical conductive portion of the X-ray tube.

In the X-ray tube disclosed in Japanese Patent Application Laid-Open No.2009-245806, the electrical resistive film is not applied onto a portionof the valve adjacent to a voltage application portion so as to maintaininsulation of the valve. However, in this structure, in order to preventa potential gradient in an area that is not covered with the electricalresistive film from being steep in a potential distribution on the outersurface of the valve, a complicated valve shape is required.

SUMMARY OF THE INVENTION

It is an object of the present invention to improve withstand voltageproperty of an X-ray generating tube by a simple structure, and furtherto provide an X-ray generating apparatus and an X-ray imaging systemhaving high reliability by using the X-ray generating tube havingimproved withstand voltage property.

According to a first embodiment of the present invention, there isprovided an X-ray generating tube, including: an insulating tube; acathode including an electron emitting source, the cathode covering oneopening of the insulating tube so that a peripheral edge portion thereofis joined to an end surface of the one opening; an anode including atarget, the anode covering the other opening of the insulating tube sothat a peripheral edge portion thereof is joined to an end surface ofthe other opening; and a resistive film disposed on an outer peripheryof the insulating tube, the resistive film having a smaller sheetresistance value than a sheet resistance value of the insulating tube.The cathode and the anode are electrically connected to each other viathe resistive film. A dark current at 100° C. when a voltage of 100 kVis applied between the cathode and the anode is 0.1 μA or more to 10 μAor less.

According to a second embodiment of the present invention, there isprovided an X-ray generating apparatus, including: the X-ray generatingtube according to the first embodiment of the present invention; and acontainer for storing the X-ray generating tube. The container includesan emission window for emitting X-ray generated by the X-ray generatingtube. A rest space inside the container storing the X-ray generatingtube is filled with insulating liquid.

According to a third embodiment of the present invention, there isprovided an X-ray imaging system, including: the X-ray generatingapparatus according to the second embodiment of the present invention;an X-ray detecting device configured to detect X-ray emitted from theX-ray generating apparatus and passing through a subject to beinvestigated; and a control device configured to control the X-raygenerating apparatus and the X-ray detecting device in a coordinatedmanner.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are schematic diagrams illustrating a structure of anX-ray generating tube according to an embodiment of the presentinvention.

FIG. 2 is a cross-sectional schematic diagram illustrating a structureof an X-ray generating apparatus according to an embodiment of thepresent invention.

FIG. 3 is a block diagram schematically illustrating a structure of anX-ray imaging system according to an embodiment of the presentinvention.

FIGS. 4A and 4B are cross-sectional schematic diagrams illustrating oneanother electrical connection form of the X-ray generating tubeaccording to the present invention.

FIG. 5 is a cross-sectional schematic diagram illustrating 2^(nd)another electrical connection form of the X-ray generating tubeaccording to the present invention.

DESCRIPTION OF THE EMBODIMENTS

Now, exemplary embodiments of the present invention are described indetail with reference to the attached drawings, but the presentinvention is not limited to these embodiments. In addition, a known orwell-known technology in the art is applied to a portion that is notparticularly illustrated or described in this specification. Further, inthe drawings to be referred to below, the same numeral or symbol denotesthe same component.

As illustrated in FIG. 1A, an X-ray generating tube 1 of the presentinvention basically includes an insulating tube 5, a cathode 2, and ananode 4. The cathode 2 covers one opening of the insulating tube 5, anda peripheral edge portion of the cathode 2 is joined to one end surface13 of the insulating tube 5. The anode 4 covers the other opening of theinsulating tube 5, and a peripheral edge portion of the anode 4 isjoined to the other end surface 13 of the insulating tube 5. Further,the cathode 2 has an electron emitting source 7, and the anode 4 has atransmissive target 3. It can be said that the anode 4 forms an endwindow of the X-ray generating tube 1. A lens electrode 8 and anextraction electrode 9 are each connected to a power supply (not shown)so that a tube voltage of the X-ray generating tube 1 is defined.

As materials of the cathode 2 and the anode 4, there are Kovar, steel,steel alloy, SUS material, and a metal such as Ag, Cu, Ti, Mn, Mo, andNi, and an alloy of these metals.

The insulating tube 5 is usually a cylindrical tube, but a tube havingan elliptical or polygonal cross section may be used in the presentinvention. As a material of the insulating tube 5, there are so-calledceramic materials such as Al₂O₃ (alumina), Si₃N₄, SiC, AlN, and ZrO₃,but any insulation material may be used.

The cathode 2, the anode 4, and the insulating tube 5 are vacuum-tightlyjoined to each other so as to form a vacuum container. As joining means,brazing, welding, or the like can be used.

As the electron emitting source 7, a hot cathode such as a tungstenfilament and an impregnated cathode, or a cold cathode such as a carbonnanotube can be used.

The target 3 may be made of a material that can emit X-rays by electronirradiation or may have a structure including an x-ray transmissivesubstrate and a target metal film formed thereon. As the x-raytransmissive substrate, it is preferred to use a substrate havingstrength capable of supporting the target metal, little absorption ofX-rays generated by the target metal, and high thermal conductivity sothat heat generated by the target metal can be rapidly dissipated. Forinstance, diamond, silicon carbide, aluminum nitride, or the like can beused.

As a material forming the target metal, it is preferred to use amaterial having a high melting point and high X-ray generationefficiency. For instance, tungsten, tantalum, molybdenum, or the likecan be used. In order to reduce absorption that occurs when thegenerated X-ray passes through the target metal, it is appropriate thatthe target metal has a thickness of a few μm to ten and a few μm.

In the X-ray generating tube 1 of the present invention, a tube voltageis applied between the cathode 2 and the anode 4 by a high voltage powersupply (not shown), and electrons generated by the electron emittingsource 7 are drawn into vacuum by the extraction electrode 8. Then, theelectrons are accelerated with the anode 4 whose potential is set to bepositive of the tube voltage with respect to the cathode 2. Further, anelectron beam 10 to be a tube current is converged by an action of thelens electrode 9, and the high energy electron beam 10 irradiates thetarget 3 made of a metal such as tungsten so that X-rays are generated.

The X-ray generating tube 1 of the present invention has a feature inthat a resistive film 6 having a lower sheet resistance value than thatof the insulating tube 5 is disposed on the outer periphery of theinsulating tube 5, and that the resistive film 6 electrically connectsthe cathode 2 and the anode 4. With this structure, electrification ofthe surface caused in the related-art insulating tube 5 can beprevented, and an excessive potential gradient is not formed between thecathode 2 and the anode 4 so that discharge generated in the X-raygenerating tube 1 is suppressed.

In the present invention, in order to obtain the effect of the resistivefilm 6, the resistive film 6 is formed so that a dark current becomes0.1 μA or more to 10 μA or less at 100° C. when a voltage of 100 kV isapplied between the cathode 2 and the anode 4. As a material of theresistive film 6, a glass material such as Kovar glass, glaze, and fritglass, or a metal oxide film can be used as long as a predeterminedresistance can be obtained.

In the present invention, it is preferred that a specific resistance ofthe insulating tube 5 at 100° C. be 1×10 Ωm or more to 1×10¹⁴ Ωm orless. Further, when the specific resistance of the insulating tube 5falls within the above-mentioned range, it is preferred thatR_(s)2/R_(s)1 be 1×10⁻⁵ or more to 1×10⁻¹ or less, where R_(s)1represents a sheet resistance value of the insulating tube 5 at 100° C.,and R_(s)2 represents a sheet resistance value of the resistive film 6at 100° C.

Next, the dark current flowing between the cathode 2 and the anode 4 at100° C. and the sheet resistance values of the insulating tube 5 and theresistive film 6 at 100° C. defined in the present invention arecalculated based on IV characteristics measured at multipletemperatures. A measuring method of the IV characteristics and acalculation method of the individual values are described below.

(1) A measuring electrode pair is a comb-like electrode pair having anelectrode interval d of 50 μm, an electrode opposing section length L of50 mm, and an electrode line width of 50 μm in an electrode opposingsection. This comb-like electrode pair is formed on a surface of theresistive film 6 of the X-ray generating tube 1 for a measuring sampleof the resistive film 6. In addition, the comb-like electrode pair isformed on an outer peripheral surface or an inner peripheral surface ofthe insulating tube 5 exposed by partially separating the resistive film6 of the X-ray generating tube 1 for a measuring sample of theinsulating tube 5. The measuring sample is placed in a chamber includinga heat stage controlled by a temperature adjustment device and ameasuring terminal connected to an IV measuring apparatus. The inside ofthe chamber can be vacuumed. Thermocouples connected to the temperatureadjustment device are adhered onto the surface of the resistive film 6and the surface of the insulating tube 5 on each of which the comb-likeelectrode pair is formed, by using a thermoset adhesive, and thecomb-like electrode pair is connected to the measuring terminal of thechamber.

(2) The inside of the chamber is evacuated to a vacuum degree of 1×10⁻⁴Pa.

(3) The inside of the chamber is heated so that the measuring sample has400° C. and is controlled so that the temperature is constant for 5minutes. Next, in the constant temperature period, a voltage V_(test)[V] is applied so as to measure a dark current I [A] flowing in theelectrode pair. The voltage V_(test) is determined by considering adistance D [m] between the cathode 2 and the anode 4, V_(a)/D [V/m]determined by the tube voltage V_(a) [V], and an inter-electrodedistance d [m] between the comb-like electrodes. In the presentinvention, the tube voltage is set to V_(a)=100×10³ V.

The measurement described above is performed with a decreasedtemperature of 200° C. or 25° C. In this embodiment, the measurement isperformed at three points while decreasing the temperature, but it ispossible to perform the measurement while increasing the temperature.However, it is preferred to perform the measurement while decreasing thetemperature because an influence of water adsorbed on the surface bybake effect can be reduced.

(4) An electrical conductivity σ(T) [S/m] is calculated based on thedark current I [A] obtained in (3) described above, a value of theapplied voltage [V], and dimensions of a conductive path (the electrodeinterval d, the electrode length L, and the film thickness t [m] of theresistive film 6 and the insulating tube 5). Here, T represents anabsolute temperature [K].

Electrical conductivities σ (673.15) at 400° C., σ (473.15) at 200° C.,and α (298.15) at 25° C. are determined based onα(T)=(I(T)/V)×(d/(L×t)).

(5) Temperature dependence data of the electrical conductivity aobtained in (4) described above is substituted into the followingArrhenius equation, to thereby determine, by using the least squaresmethod, activation energy Ea [eV] of the electrical conductivity a andan electrical conductivity σ₀ [S/m] under a theoretical temperatureinfinity condition.

σ(T)[S/m]=σ ₀×exp(−e×Ea/kT)

In the above equation, e represents a unit charge (electron quantum,1.9×10⁻¹⁹ [C]), and k represents a Boltzmann constant (1.38×10⁻²³[J/K]).

An electrical conductivity σ (373.15) at 100° C. as an evaluationcriterion in the present invention is determined, and is converted intoσ(T)=σ_(s)(T)×t so as to determine a sheet electrical conductivity σ_(s)[S·□] at 100° C. Further, the reciprocal of the sheet electricalconductivity σ_(s) is determined as a sheet resistance value R_(s) [Ω/□]at 100° C.

In addition, as to the dark current in the resistive film, theelectrical conductivity σ (373.15) at 100° C. determined based on theabove-mentioned Arrhenius equation is substituted into the equationσ(T)=(I(T)/V)×(d/(L×t)) described above. In this manner, the darkcurrent I [A] at 100° C. is obtained.

In the present invention, as to the electrical connection between theresistive film 6 and the cathode 2 as well as the anode 4, it ispreferred that, as illustrated in FIG. 1A, at least one of the cathode 2and the anode 4 include on the peripheral edge portion thereof anextending portion protruding in a longitudinal direction of theinsulating tube 5, the extending portion being connected to theresistive film 6. As the connection means, the cathode 2 or the anode 4as a metal member may be mechanically pressed to contact with theresistive film 6, or a conductive paste may be used for connection. Theconnection means is not particularly limited as long as electricalconnection can be established.

In addition, in FIG. 1A, the resistive film 6 is sandwiched between theinsulating tube 5 and the extending portion of the cathode 2 or theanode 4. However, the extending portion may be sandwiched between theinsulating tube 5 and the resistive film 6. In addition, as illustratedin FIG. 1B, it is also preferred to adopt a structure in which theextending portion of the cathode 2 or the anode 4 is sandwiched betweenthe insulating tube 5 and the resistive film 6, and an end portion ofthe resistive film 6 connected to the extending portion is coated withconductive paste 11.

In the embodiment illustrated in FIG. 1A, an electrical connectionportion 12 between the resistive film 6 and the anode 4 or the cathode 2is positioned on an outer peripheral portion of the insulating tube 5.It is sufficient that the X-ray generating tube 1 of the presentinvention includes the resistive film 6 disposed at least on the outerperipheral portion of the insulating tube 5 so that the resistive film 6is electrically connected to the anode 4 and the cathode 2. Otherelectrical connection forms are included in the present invention.

For instance, as in each embodiment illustrated in FIG. 4A, FIG. 4B, orFIG. 5, the end surface 13 of the insulating tube 5 may include theelectrical connection portion 12 between the resistive film 6 and theanode 4 or the cathode 2. Each embodiment illustrated in FIG. 4A, FIG.4B, or FIG. 5 is an embodiment in which the anode 4 and the cathode 2without the extending portion are connected to the insulating tube 5including the resistive film 6 formed on the outer periphery and on theend surface 13 with silver braze.

The embodiment illustrated in FIG. 4A is an embodiment in which a radiusof the outer periphery of the insulating tube 5 is the same as a radiusof the outer periphery of the anode 4 and the cathode 2, and theresistive film 6 is connected to the anode 4 and the cathode 2 on theopposing end surfaces 13 of the insulating tube 5.

In addition, the embodiment illustrated in FIG. 4B is an embodiment inwhich the radius of the outer periphery of the anode 4 and the cathode 2is larger than the radius of the outer periphery of the insulating tube5, and the resistive film 6 is connected to the anode 4 and the cathode2 on the opposing end surfaces 13 of the insulating tube 5. In thisembodiment, the outer peripheral portions of the cathode 2 and the anode4 protrude outward from the outer peripheral portion of the insulatingtube 5 in the radial direction of the insulating tube 5.

In addition, the embodiment illustrated in FIG. 5 is an embodiment inwhich the radius of the outer periphery of the anode 4 and the cathode 2is smaller than the radius of the outer periphery of the insulating tube5, and the resistive film 6 is connected to the anode 4 and the cathode2 on the opposing end surfaces 13 of the insulating tube 5. In thisembodiment, the outer peripheral portions of the cathode 2 and the anode4 protrude inward from the outer peripheral portion of the insulatingtube 5 in the radial direction of the insulating tube 5.

In the embodiments illustrated in FIG. 4A, FIG. 4B, and FIG. 5, adistance between the anode 4 and the cathode 2 on the outer peripheralportion of the insulating tube 5 can be larger than that in theembodiment illustrated in FIG. 1A, and hence an average electric fieldin the tube axis direction on the outer peripheral portion can bereduced.

The inside of the X-ray generating tube 1 can be vacuumed by evacuatingthe air with use of an exhausting tube (not shown) and then by sealingthe exhausting tube. It is possible to dispose a getter (not shown) inthe X-ray generating tube 1 manufactured in this way for maintainingmore appropriate vacuum.

Next, an X-ray generating apparatus of the present invention isdescribed. FIG. 2 is a cross-sectional schematic diagram illustrating anexample of a structure of the X-ray generating apparatus including theX-ray generating tube 1 of FIG. 1A. As illustrated in FIG. 2, the X-raygenerating apparatus of the present invention includes the X-raygenerating tube 1 of the present invention and a container 21 forstoring the X-ray generating tube 1. A rest space in the container 21 isfilled with insulating liquid 23. In addition, the container 21 includesan X-ray emission window 22 for emitting the X-ray generated by theX-ray generating tube 1.

A drive circuit 16 including a circuit substrate and an insulatingtransformer (not shown) may be disposed inside the container 21. In thecase where the drive circuit 16 is disposed, for instance, apredetermined voltage signal is applied to the X-ray generating tube 1from the drive circuit 16 via wirings (not shown) so that generation ofthe X-ray can be controlled.

The container 21 only needs to have strength sufficient for a containerand is made of a metal or plastic material. The container 21 includesthe emission window 22 that transmits the X-ray for emitting the X-rayto the outside of the container 21. The X-ray emitted by the X-raygenerating tube 1 passes through this emission window 22 and is emittedto the outside. The emission window 22 is made of glass, aluminum,beryllium, or the like.

It is preferred that the insulating liquid 23 have high electricalinsulating property and high cooling performance, and be hardly degradedby heat. For instance, electrical insulation oil such as silicone oil,transformer oil, or fluorine-containing oil, and fluorine-containinginsulating liquid such as hydro fluoro ether can be used.

Next, an X-ray imaging system according to an embodiment of the presentinvention is described with reference to FIG. 3.

As illustrated in FIG. 3, an X-ray generating apparatus 100 according tothe present invention includes a movable diaphragm unit 31 disposed at aportion corresponding to the X-ray emission window 22 as necessary. Themovable diaphragm unit 31 has a function of adjusting a radiation fieldof an X-ray 101 radiated from the X-ray generating apparatus 100. Inaddition, it is possible to use the movable diaphragm unit 31 having anadditional function to perform simulation display of the radiation fieldof the X-ray using visible light.

A system control device 202 controls the X-ray generating apparatus 100and an X-ray detecting device 201 in a coordinated manner. The drivecircuit 16 outputs various control signals to the X-ray generating tube1 under control by the system control device 202. This control signalcontrols a radiation state of the X-ray 101 emitted from the X-raygenerating apparatus 100. The X-ray 101 emitted from the X-raygenerating apparatus 100 passes through a subject to be investigated 204and is detected by a detector 206. The detector 206 converts thedetected X-ray into an image signal and outputs the image signal to asignal processing portion 205. Under control by the system controldevice 202, the signal processing portion 205 performs predeterminedsignal processing on the image signal and outputs the processed imagesignal to the system control device 202. The system control device 202generates a display signal for controlling a display device 203 todisplay an image based on the processed image signal and outputs thedisplay signal to the display device 203. The display device 203displays an image based on the display signal as a photographed image ofthe subject to be investigated 204 on a screen.

The X-ray imaging system can be used for nondestructive inspection of anindustrial product or pathological diagnosis of a human body or ananimal body.

Example 1

The X-ray generating tube 1 illustrated in FIG. 1A was manufactured.

Slurry liquid made of a solution containing powder Kovar glass andacetic acid as main ingredients was sprayed and applied onto the outerperipheral surface of the cylindrical insulating tube 5 made or alumina,and was melted and welded at a glazing temperature of 1,000° C. so as toform the resistive film 6.

Next, the insulating tube 5 was sealed with the anode 4 made of Kovarmetal with the target 3 mounted thereon and the cathode 2 made of Kovarmetal with the electron emitting source 7 by using silver braze at 900°C. After that, the extending portions of the anode 4 and the cathode 2were swaged toward the insulating tube 5 by an external force so thatthe anode 4 and the cathode 2 were electrically connected to theresistive film 6 at the extending portions overlapping the end portionsof the resistive film 6.

An electric field was applied to the X-ray generating tube 1 having theabove-mentioned structure in the creepage direction of the insulatingtube 5 and the resistive film 6 so as to measure the IV characteristicsby the measuring method described above. As a result, it was confirmedthat the sheet resistance value of the resistive film 6 at 100° C. was5×10¹¹Ω/□, and the sheet resistance value of the insulating tube 5 at100° C. was 1×10¹⁶Ω/□.

As a result, it was found that a dark current flowing between thecathode 2 and the anode 4 when a voltage of 100 kV was applied was 0.5μA. It was confirmed that the dark current was sufficiently smaller thanthe tube current (10 mA) caused by the electron beam for generating anX-ray by the X-ray generating tube 1 and had no problem.

Further, a voltage of 100 kV was applied between the cathode 2 and theanode 4 so as to conduct an experiment in X-ray generation at a tubecurrent of 10 mA. There was no occurrence of discharge, and it waspossible to obtain stable X-ray radiation. In this example, there isalso an effect of relieving concentration of the electric field to belower at a triple point formed of the cathode 2, the resistive film 6,and the insulating liquid 23 than at a triple point formed at an endportion on the electrical insulation portion of the related-artelectrical resistive film.

In addition, the reason why the X-ray generating tube 1 of this examplewas able to radiate a stable X-ray is estimated as follows. A variationand fluctuation of potential gradient in the inner space of the X-raygenerating tube 1 sandwiched between the cathode 2 and the anode 4 issuppressed by a current field in the tube axis direction defined by thelow resistive film 6. As a result, it is estimated that a locus of theelectron beam 10 emitted from the electron emitting source 7 wasstabilized.

Example 2

The X-ray generating tube 1 illustrated in FIG. 1B was manufactured.

The cylindrical insulating tube 5 made of alumina was sealed with theanode 4 made of Kovar metal with the target 3 mounted thereon and thecathode 2 made of Kovar metal with the electron emitting source 7mounted thereon by using copper braze at 1,050° C. After that, theresistive film 6 similar to that of Example 1 was formed on the outerperipheral surface of the insulating tube 5 so that each end thereofoverlapped with a portion of the extending portion of the anode 4 or thecathode 2. Further, the conductive paste 11 was applied onto theportions of the resistive film 6 where the extending portion of theanode 4 or the cathode 2 was overlapped so that end portions of theresistive film 6 were covered.

Next, the IV characteristics of the insulating tube 5 and the resistivefilm 6 were measured for the X-ray generating tube 1 having thestructure described above similarly to Example 1, the sheet resistancevalue and the dark current at 100° C. were calculated, and theexperiment in X-ray generation was conducted. As a result, the sameappropriate result as in Example 1 was obtained. A contact angle of ametal with respect to the connection cross section is milder at a triplepoint formed of the conductive paste 11, the resistive film 6, and theinsulating liquid 23 of this example than at a triple point at which theresistive film 6 is directly connected to the cathode 2 or the anode 4.Therefore, there is also an effect of relieving concentration of theelectric field.

In addition, also in the X-ray generating tube 1 of this example, it waspossible to obtain stable radiation of an X-ray.

According to the present invention, the surface of the insulating tubeis covered with the resistive film having a lower sheet resistance valuethan the insulating tube, and the cathode and the anode are electricallyconnected. Thus, electrification on the surface of the conventionalinsulating tube can be prevented, and excessive potential gradient canbe relieved. Therefore, it is possible to provide the X-ray generatingtube with a simple structure, which has the improved withstand voltageobtained by suppressing occurrence of discharge. In addition, it ispossible to provide the X-ray generating apparatus and the X-ray imagingsystem having high reliability by using the X-ray generating tube.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Applications No.2013-118467, filed Jun. 5, 2013, and No. 2013-246985, filed Nov. 29,2013, which are hereby incorporated by reference herein in theirentirety.

1.-17. (canceled)
 18. An X-ray generating tube comprising: an insulatingtube extending along a tube axis and having a pair of tube end portions;a cathode having an electron emitting source, the cathode being securedto one of the tube end portions; an anode having a target, the anodebeing secured to the other of the tube end portions; and a resistivefilm disposed on an outer periphery of the insulating tube, theresistive film having a lower sheet resistance than a sheet resistanceof the insulating tube, wherein each of the cathode and the anode isconnected to the resistive film, respectively.
 19. The X-ray generatingtube according to claim 18, wherein a dark current at 100° C. when avoltage of 100 kV is applied between the cathode and the anode is 0.1 μAor more to 10 μA or less.
 20. The X-ray generating tube according toclaim 18, wherein at least one of the cathode and the anode comprises anextending portion protruding from the peripheral edge portion in alongitudinal direction of the insulating tube; and wherein the resistivefilm is connected to the extending portion.
 21. The X-ray generatingtube according to claim 20, wherein the resistive film is sandwichedbetween the extending portion and the insulating tube.
 22. The X-raygenerating tube according to claim 20, wherein the extending portion issandwiched between the resistive film and the insulating tube.
 23. TheX-ray generating tube according to claim 22, wherein an end portion ofthe resistive film joined to the extending portion is covered with aconductive paste.
 24. The X-ray generating tube according to claim 20,wherein an electrical connection portion between the resistive film andthe at least one of the cathode and the anode is positioned on an outerperipheral portion of the insulating tube.
 25. The X-ray generating tubeaccording to claim 18, wherein an electrical connection portion betweenthe resistive film and at least one of the cathode and the anode ispositioned on an end surface of the insulating tube.
 26. The X-raygenerating tube according to claim 18, wherein the insulating tube ismade of ceramic, and the resistive film is made of a glass material. 27.The X-ray generating tube according to claim 26, wherein the insulatingtube is made of alumina, and the resistive film is made of glaze. 28.The X-ray generating tube according to claim 18, wherein a specificresistance of the insulating tube at 100° C. is 1×10 Ωm or more to1×10¹⁴ Ωm or less; and wherein R_(s)2/R_(s)1 is 1×10⁻⁵ or more to 1×10⁻¹or less, where R_(s)1 represents a sheet resistance value of theinsulating tube at 100° C., and R₅2 represents a sheet resistance valueof the resistive film at 100° C.
 29. The X-ray generating tube accordingto claim 18, wherein the outer periphery of the insulating tube is anouter periphery of the X-ray generating tube.
 30. The X-ray generatingtube according to claim 18, wherein the resistive film defines a spaceelectric field between the cathode and the anode.
 31. The X-raygenerating tube according to claim 18, wherein the anode comprises anend window including a transmissive target.
 32. An X-ray generatingapparatus comprising: the X-ray generating tube according to claim 18;and a container for storing the X-ray generating tube, wherein thecontainer comprises an emission window for emitting X-ray generated bythe X-ray generating tube, and a rest space inside the container storingthe X-ray generating tube is filled with insulating liquid.
 33. TheX-ray generating apparatus according to claim 32, wherein the insulatingliquid comprises at least one of silicone oil, transformer oil, andfluorine-containing oil.
 34. An X-ray imaging system comprising: theX-ray generating apparatus according to claim 32; an X-ray detectingdevice configured to detect X-ray emitted from the X-ray generatingapparatus and passing through a subject to be investigated; and acontrol device configured to control the X-ray generating apparatus andthe X-ray detecting device in a coordinated manner.
 35. An X-raygenerating tube comprising: an insulating tube extending along a tubeaxis and having a pair of tube end portions; a cathode having anelectron emitting source, the cathode being secured to one of the tubeend portions; an anode having a target, the anode being secured to theother of the tube end portions; and a resistive film disposed on anouter periphery of the insulating tube, the resistive film having alower sheet resistance than a sheet resistance of the insulating tube,wherein the resistive film is electrically connected to each of thecathode and the anode such that a variation and fluctuation of potentialgradient in the inner space of the X-ray generating tube is suppressedby a current field along the tube axis direction defined by theresistive film.
 36. The X-ray generating tube according to claim 35,wherein a dark current at 100° C. when a voltage of 100 kV is appliedbetween the cathode and the anode is 0.1 μA or more to 10 μA or less.37. The X-ray generating tube according to claim 35, wherein at leastone of the cathode and the anode comprises an extending portionprotruding from the peripheral edge portion in a longitudinal directionof the insulating tube; and wherein the resistive film is connected tothe extending portion.
 38. The X-ray generating tube according to claim37, wherein the resistive film is sandwiched between the extendingportion and the insulating tube.
 39. The X-ray generating tube accordingto claim 37, wherein the extending portion is sandwiched between theresistive film and the insulating tube.
 40. The X-ray generating tubeaccording to claim 39, wherein an end portion of the resistive filmjoined to the extending portion is covered with a conductive paste. 41.The X-ray generating tube according to claim 37, wherein an electricalconnection portion between the resistive film and the at least one ofthe cathode and the anode is positioned on an outer peripheral portionof the insulating tube.
 42. The X-ray generating tube according to claim35, wherein an electrical connection portion between the resistive filmand at least one of the cathode and the anode is positioned on an endsurface of the insulating tube.
 43. The X-ray generating tube accordingto claim 35, wherein the insulating tube is made of ceramic, and theresistive film is made of a glass material.
 44. The X-ray generatingtube according to claim 43, wherein the insulating tube is made ofalumina, and the resistive film is made of glaze.
 45. The X-raygenerating tube according to claim 35, wherein a specific resistance ofthe insulating tube at 100° C. is 1×10 Ωm or more to 1×10¹⁴ Ωm or less;and wherein R_(s)2/R_(s)1 is 1×10⁻⁵ or more to 1×10⁻¹ or less, whereR_(s)1 represents a sheet resistance value of the insulating tube at100° C., and R_(s)2 represents a sheet resistance value of the resistivefilm at 100° C.
 46. The X-ray generating tube according to claim 35,wherein the outer periphery of the insulating tube is an outer peripheryof the X-ray generating tube.
 47. The X-ray generating tube according toclaim 35, wherein the resistive film defines a space electric fieldbetween the cathode and the anode.
 48. The X-ray generating tubeaccording to claim 35, wherein the anode comprises an end windowincluding a transmissive target.
 49. An X-ray generating apparatuscomprising: the X-ray generating tube according to claim 35; and acontainer for storing the X-ray generating tube, wherein the containercomprises an emission window for emitting X-ray generated by the X-raygenerating tube, and a rest space inside the container storing the X-raygenerating tube is filled with insulating liquid.
 50. The X-raygenerating apparatus according to claim 49, wherein the insulatingliquid comprises at least one of silicone oil, transformer oil, andfluorine-containing oil.
 51. An X-ray imaging system comprising: theX-ray generating apparatus according to claim 49; an X-ray detectingdevice configured to detect X-ray emitted from the X-ray generatingapparatus and passing through a subject to be investigated; and acontrol device configured to control the X-ray generating apparatus andthe X-ray detecting device in a coordinated manner.